Stabilized Strength Development and Enhanced Heavy Metal Solidification in Alkali-Activated MSWIFA via Biochar Incorporation

Abstract

This study employed biochar (BC) as an additive in a ternary system comprising municipal solid waste incineration fly ash (MSWIFA), slag, and BC to evaluate its effects on mechanical strength, hydration, and heavy metal immobilization. The results indicate that water washing and pyrolysis enhance FA reactivity, while the porous structure of BC promotes nucleation of hydration products, refines the pore structure, and increases the proportion of harmless pores. Compared to 3-day values, specimens with 6% BC and pyrolyzed FA exhibited increases of 11.3% in compressive strength and 23.0% in flexural strength at 56 days. Leaching of Cr, Cd, and Cu at 28 days was reduced by 17%–63%, meeting environmental standards. BC incorporation effectively mitigates strength degradation and heavy-metal leaching risks in MSWIFA-based alkali-activated materials, highlighting its role in improving durability and environmental safety.